1. Field of the Invention
The present invention relates to an in-cylinder pressure detection device for an internal combustion engine, which detects in-cylinder pressure defined as pressure within a cylinder of the engine, a method of detecting the in-cylinder pressure, and an engine control unit.
2. Description of the Related Art
Conventionally, there has been disclosed an in-cylinder pressure detection device e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H09-133042. This in-cylinder pressure detection device has in-cylinder pressure sensors provided for a plurality of (four) cylinders, respectively. Each of the in-cylinder pressure sensors is formed by an annular piezoelectric element, and is screwed into a cylinder head together with a spark plug to be nipped between the spark plug and the cylinder head. In the in-cylinder pressure detection device, the average value of four detection values each obtained at a predetermined crank angle position of an associated cylinder during a compression stroke is calculated, and a relation between each detection value and the calculated average value is obtained and stored on a cylinder-by-cylinder basis. Then, an average value corresponding to a current detection value is obtained from the stored relations, and the obtained average value is used as a value indicative of a final in-cylinder pressure. Thus, variations in the detection values associated with the respective cylinders are compensated for.
An in-cylinder pressure sensor of the above-mentioned type is screwed into a cylinder head together with a spark plug, as mentioned above, so that even when in-cylinder pressure decreases, pressure remains on a screw part of the spark plug, which makes a detection value detected by the sensor larger than an actual in-cylinder pressure by an amount corresponding to the remaining pressure (hereinafter referred to as “the hysteresis amount”). The above-described in-cylinder pressure detection device, which simply uses the average value of detection values from the respective in-cylinder pressure sensors as a detected in-cylinder pressure, cannot accommodate differences between the detection values and the actual in-cylinder pressure, which causes degradation of accuracy of the detected in-cylinder pressure.